Conventionally, an apparatus for detecting a decrease in tire air pressure uses a principle according to which a tire having a decreased pressure has a smaller outer diameter (tire dynamic loaded radius) than those of tires having a normal air pressure and thus has an increased angular velocity (wheel speed) when compared with other normal tires. For example, in the case of a method of detecting a decrease in air pressure based on the relative difference in the tire wheel speed, the following determination value DEL is used.DEL={(V1+V4)/2−(V2+V3)/2}/{(V1+V2+V3+V4)/4}×100(%)
When the absolute value of this DEL exceeds an alarm threshold value, a decreased pressure alarm is issued (see Patent Literature 1 for example). In the formula, V1 to V4 denote the wheel speeds of a left-front wheel tire, a right-front wheel tire, a left-rear wheel tire, and a right-rear wheel tire, respectively.
However, although the above determination value DEL can tell that any of the tires (one or two or more tires) has/have a decreased pressure, the above determination value DEL cannot tell which of the tires has a decreased pressure. A continued running with a tire having a decreased pressure equal to or higher than a predetermined amount causes a disadvantage that the fuel cost is deteriorated due to an increased tire rolling resistance to thereby consequently cause a burst. Thus, when such a decreased pressure occurs, a driver must adjust the tire air pressure in a nearby service station for example.
However, the failure to specify the position of a tire having a decreased pressure may cause a risk of a long time to adjust the air pressure.
To solve this, various methods have been suggested by which a tire having a decreased air pressure can be specified (see Patent Literatures 2 to 3 for example).
According to the methods disclosed in Patent Literatures 2 and 3, three determination values DEL1, DEL2, and DEL3 are used so that a tire having a decreased pressure can be specified based on the relation in the magnitude between the respective determination values and a predetermined threshold value. The determination value DEL1 is obtained by dividing a difference between the sums of the wheel rotation information of pairs of wheel tires on two diagonal lines by a predetermined average wheel speed. The determination value DEL2 is obtained by dividing a difference between the sum of the wheel rotation information of the front wheel tires and the sum of the wheel rotation information of the rear wheel tires by the predetermined average wheel speed. The determination value DEL3 is obtained by dividing the difference between the sum of the wheel rotation information of the right wheel tires and the sum of the wheel rotation information of the left wheel tires by the predetermined average wheel speed.